This invention generally relates to a process, and an apparatus for causing the development of images in electrostatographic systems. More specifically, the present invention is directed to an improved process, and an improved apparatus for accomplishing the development of electrostatic latent images, by providing a development zone encompassed by a moving deflected flexible imaging member, and a moving transporting member. The flexible imaging member is deflected by electrically insulating developer particles, comprised of insulating toner particles, and insulating magnetic carrier particles contained in the development zone, which deflection, together with the relative movement of said members, is primarily responsible for the agitation and movement of the developer particles. Such a process, and apparatus allows the continual development of high quality images, including the efficient and effective development of solid areas.
The development of images by electrostatographic means is well known. Generally in these systems, toner particles are applied to an electrostatic latent image by various methods including cascade development, reference U.S. Pat. No. 3,618,552, magnetic brush development, reference U.S. Pat. Nos. 2,874,063, 3,251,706, and 3,357,402, powder cloud development, reference U.S. Pat. No. 2,217,776, and touchdown development, reference U.S. Pat. No. 3,166,432. Cascade development and powder cloud development methods have been found to be especially well suited for the development of line images common to business documents, however, images containing solid areas are not faithfully reproduced by these methods. Magnetic brush development systems, however, provided an improved method for reproducing both line images, and solid areas.
In magnetic brush development systems, it is usually desirable to attempt to regulate the thickness of the developer composition, which is transported on a roller, by moving the roller past a metering blade. The adjustment of the metering blade is important, since in the development zone the flow of developer material is determined by a narrow restrictive opening situated between the transport roller and the imaging surface. Accordingly, in order to provide sufficient toner particles to the imaging surface, it is generally necessary to compress the developer bristles, thereby allowing toner particles adhering to the carrier particles near the ends of the bristle to be available for development. Any variation, or non-uniformity in the amount of developer metered onto the transport roller, or into the spacing between the transport roller and imaging member can result in undesired developer flow, and non-uniform image development. Non-uniform development is usually minimized by carefully controlling developer runout on the transport roller, and on the imaging member, and by providing a means for side-to-side adjustment in the relative positions of the metering blade, development roller and imaging member.
Moderate solid area development with magnetic brush is usually achieved by transporting the developer composition on a roller at a speed that exceeds the process speed of the image bearing member. At high process speeds the development-transport roller speed is limited by centrifugal forces, which forces cause the developer material to be removed from the roller. Thus, in order to obtain moderate solid area development at high process speeds, the use of multiple development rolls is necessary for increased developability.
The developer materials presently used in magnetic brush development differ widely in their electrical conductivity, thus at one extreme in conductivity, such materials can be insulating, in that a low electrical current is measured when a voltage is applied across the developer. Solid area development with insulating developer compositions is accomplished by metering a thin layer of developer onto a development roll, which is in close proximity to an image bearing member, the development roll functioning as an electrode, and thus increasing the electrostatic force acting on the toner particles. In these systems, the spacing between the image bearing member, and the development roller must be controlled to ensure proper developer flow, and uniform solid area development, the minimum average spacing generally being typically greater than 1.5 millimeters.
Insulating developer compositions can be rendered conductive by utilizing a magnetic carrier material which supports a high electric current flow in response to an applied potential. Generally, the conductivity of developer compositions depends on a number of factors including the conductivity properties of the magnetic carrier, the concentration of the toner particles, the magnetic field strength, the spacing between the image bearing member and the development roll, and developer degradation due to toner smearing on the carrier particles. Also, when insulative toner particles are permanently bonded to a conductive carrier, the conductivity decreases to a critical value below which solid area development becomes inadequate, however, within certain limits the process and material parameters can be adjusted somewhat to recover the decrease in solid area developability.
When employing conductive developer materials in electrostatographic imaging systems, the development electrode member is maintained at a close effective distance from the image bearing member, and a high electrostatic force acts only on those toner particles which are adjacent to the image bearing member. Accordingly, since the electrostatic force for development in such systems is not strongly dependent on the developer layer thickness, the uniformity of solid area development is improved despite variations in the spacing between the image bearing member and the development roller member. More specifically, for example, in magnetic brush development systems utilizing conductive developer materials, solid area deposition is not limited by a layer of net-charged developer near the imaging member, since this charge is dissipated by conduction to a development roller. The solid area deposition is, however, limited by image field neutralization; provided there is sufficient toner available at the ends of the developer brush, which toner supply is limited to the ends or tips of the bristles, since toner cannot be extracted from the bulk of the developer mixture; wherein high developer conductivity collapses the electric field within the developer at any location, and confines it to a region between the latent image and the developer. For either insulative or conductive developer, solid area deposition is limited by toner supply at low toner concentrations, and the toner supply is limited to a layer of carrier material adjacent to the image bearing member, since the magnetic field stiffens the developer, and hinders developer mixing in the development zone.
In the above-described systems, undesirable degradation or deterioration of the developer particles results. This is generally caused by a variety of factors, including for example, the frequency of collisions between adjacent carrier particles contained in the developer composition, which collisions adversely affect the developer conductivity, and the triboelectric charging relationships between the toner particles and magnetic carrier particles. Thus, for example, a decrease in the triboelectric charge on the toner particles causes an increase in solid area development, and an increase in the amount of toner particles that are deposited in the background, or normally white areas of the image, accordingly, in order to maintain the original image quality in such situations, the triboelectric charge on the toner particles is increased, by reducing the concentration of such particles in the developer composition mixture. Also, when the toner charge, and toner concentration decreases, the developer material must be replaced in order to obtain images with acceptable solid areas and decreased background.
While several improved types of toner and carrier materials, as well as processes have been developed for the purpose of developing images, difficulties continue to be encountered in the design of a simple, inexpensive, and reliable two-component development system which will provide a high solid area development rate, low background deposition, and long term stability. The present magnetic brush systems are inherently inefficient primarily since only a small fraction of the toner transported through the development zone is accessible for deposition onto the image bearing member. For insulative developer, the solid area deposition is limited by a layer of net-charged carrier particles produced by toner development onto a precharged imaging member. Since the developer entering the development zone has a neutral charge, deposition of charged toner onto the imaging member produces a layer of oppositely charged developer which opposes further toner deposition. Also, the net electrostatic force due to the charged image member, and the net-charged developer layer becomes zero for that toner between the developer and the electrostatic latent image of the imaging member, and a collapse in the electrostatic force, or the electric field acting on the charged toner, occurs even though the toner charge deposited on the photoreceptor does not neutralize the image charge. Image field neutralization can occur, however, if there is a sufficiently high developer flow rate, and multiple development rollers. Image field neutralization results when the potential due to a layer of charged toner deposited on the imaging member is equal but opposite to the potential due to the charged imaging member. In the absence of a bias on the development roller, image neutralization produces a zero development electric field, and since the toner layer is of finite thickness, the charge density of the toner layer is less than the image charge density. Should the thickness of the charged toner layer be much less than the imaging member, image field neutralization occurs when the toner charge density neutralizes the image charge density.
Accordingly, there continues to be a need for apparatus and processes which will improve the quality of the images produced, particularly in electrostatographic systems, such as xerographic imaging systems, which are simple and economical to operate; and which result in reproducible high quality images, including both line copy and solid area image development. Additionally, there continues to be a need for the provision of an apparatus, and process wherein background development is substantially eliminated, and where the life of the developer composition is increased.